Because of their high energy density, lithium ion secondary batteries are increasingly used in recent years as portable rechargeable power sources for laptop computers, mobile phones, digital cameras, digital video cameras, and the like. Also great efforts are devoted to the development of lithium ion secondary batteries and electric double-layer capacitors using non-aqueous electrolytic solution, as auxiliary power sources for electric and hybrid automobiles which are desired to reach a practically acceptable level as environment-friendly automobiles.
The lithium ion secondary batteries, albeit their high performance, are not satisfactory with respect to discharge characteristics in a rigorous environment, especially low-temperature environment, and discharge characteristics at high output levels requiring a large quantity of electricity within a short duration of time. On the other hand, the electric double-layer capacitors suffer from problems including insufficient withstand voltages and a decline with time of their electric capacity. Most batteries use non-aqueous electrolytic solutions based on low-flash-point solvents, typically dimethyl carbonate and diethyl carbonate. In case of thermal runaway in the battery, the electrolytic solution will vaporize and be decomposed, imposing the risk of battery rupture and ignition. Then, IC circuits are generally incorporated in the batteries as means for breaking currents under abnormal conditions, and safety valves are also incorporated for avoiding any rise of the battery internal pressure by the evolution of hydrocarbon gases. It is thus desired to further elaborate the electrolytic solutions for the purposes of safety improvement, weight reduction, and cost reduction.
Under the circumstances, polyether-modified siloxanes are of great interest because they are chemically stable and compatible with electrolytic solutions. Due to their ability to dissolve electrolytes such as LiPF6 therein thoroughly and their inherent surface activity, the polyether-modified siloxanes are effective in improving the wetting of electrodes or separators. It is also known that adding only a few percents of polyether-modified siloxane to electrolytic solutions improves the charge/discharge cycle performance. However, these effects are yet insufficient. Besides, the polyether-modified siloxanes lack thermal stability and additionally, have a relatively high melting point so that they encounter some problems during low-temperature service. It would be desirable to have additives which are more stable and more compatible with electrolytic solutions.
Reference should be made to JP-A 11-214032 and JP-A 2000-58123 both corresponding to U.S. Pat. No. 6,124,062, JP-A 2001-110455, and JP-A 2003-142157.